Narrow-headed ant colonies can contain several queens and about 1,000 workers.
Photograph: John Walters

A soggy mound amid the grass stopped me in my tracks. I had spent the best part of an hour searching the heathland reserve, eyes to the ground, before I chanced across it close to a clump of gorse. It was nothing much to look at, admittedly. After a winter of heavy downpours the dome-shaped structure covered with tiny snippets of vegetation had slumped so that it resembled a spadeful of old lawn clippings. Yet beneath the bedraggled thatch was buried treasure: an exceptionally rare colony of narrow-headed ants (Formica exsecta) toughing out the colder months hidden from view.

Shaped like tiny lopsided dumbbells, these territorial ants, named for the distinctive notch in the back of their head, play an important role in maintaining heathland through their foraging behaviour. Colonies can contain several queens along with about 1,000 workers, each armed with strong mandibles and capable of engaging in chemical warfare by firing jets of formic acid from their rear to deter predators.

Despite this impressive weaponry, however, these centimetre-long wood ants are defenceless in the face of habitat loss. The deterioration of the heaths and rough grassland on which they depend has seen populations that were once widespread across the south and west gradually fragment and die out. The nest I had found, along with a few others dotted about this single field in south Devon, are all that remain in England. The next closest are in the Scottish Highlands.

The invertebrate charity Buglife and Devon Wildlife Trust are now working to re-establish lost colonies in south Devon as part of a national “Back from the Brink” initiative. Earlier in the day I had joined project officer Stephen Carroll and volunteers at nearby Bovey Heathfield reserve, clearing scrub in preparation for their planned return. “This project is the best, and perhaps last, serious chance we have to secure the future of this species in England,” Carroll said.

The nest of a narrow-headed ant colony in a south Devon field, its last remaining site in England. Photograph: Charlie Elder

I left the precious nest undisturbed, rare life concealed within it like a beating heart, and trod lightly back across the field, encouraged that this underground army of miniature eco-warriors was being given a fighting chance.